Ink jet printing is a well-known technique by which printing is accomplished without contact between the printing device and the substrate on which the printed characters are deposited. Briefly described, ink jet printing involves the technique of projecting a stream of ink droplets to a surface and controlling the direction of the stream, e.g., electronically, so that the droplets are caused to form the desired printed image on that surface. This technique of noncontact printing is well suited for application of characters onto a variety of surfaces including porous and non-porous surfaces.
Reviews of various aspects of ink jet printing can be found in these publications: Kuhn et al., Scientific American, April, 1979, 162-178; and Keeling, Phys. Technol., 12(5), 196-303 (1981). Various ink jet apparatuses are described in the following U.S. Patents: U.S. Pat. Nos. 3,060,429, 3,298,030, 3,373,437, 3,416,153, and 3,673,601.
In general, an ink jet ink composition must meet certain rigid requirements to be useful in ink jet printing operations. These relate to viscosity, resistivity, solubility, compatibility of components, and wettability of the substrate. Further, the ink must be quick-drying and smear resistant, must be capable of passing through the ink jet nozzle without clogging, and must permit rapid cleanup of the machine components with minimum effort. In addition, the jet ink composition should provide printed images that adhere well to the substrates, particularly non-porous substrates. Those of ordinary skill in the art know of challenges that non-porous substrates pose in achieving image adhesion.
Ink jet ink compositions have been disclosed. They generally contain an ink carrier, a binder resin, a colorant, and other optional additives. The carrier can be an aqueous or non-aqueous solvent. For jet printing on non-porous substrates such as glass, ceramics, metal or plastics, ink jet ink compositions have been prepared mostly using non-aqueous solvents.
Thus, ink jet inks for non-porous substrates are generally based on methanol and methyl ethyl ketone (MEK). These solvents offer fast drying jet inks and have the ability to dissolve or disperse binder resins and colorants. Additionally, these solvents have the ability to ionize conducting agents and thus provide good conductivity, especially for continuous ink jet printing operation. Thus, for example, U.S. Pat. No. 4,834,799 discloses a jet ink composition suitable for jet printing onto vinyl substrates which contain vinyl polymers and plasticizers. The composition is formulated to include a resin component having a pendant amino group such as a copolymer of methyl methacrylate and dimethylaminoethyl methacrylate, a dye, and a carrier. The carrier typically comprises a mixture of a lower alcohol such as methanol and a lower ketone such as MEK.
However, in view of the increasing public awareness and governmental regulations concerning the safety of MEK and methanol, it has become important to develop jet inks that do not contain a significant amount of MEK or methanol. To achieve this goal, ethanol based jet inks have been proposed. For example, UK Patent Application GB 2 286 402 discloses a jet ink for continuous ink jet printers containing ethanol as the liquid vehicle and a polyamide resin as the binder resin. Although ethanol based jet inks have served with some success, certain ethanol based inks do not dry fast enough for many ink jet applications due to the slow evaporation of ethanol.
Thus, there exists a need for a jet ink composition suitable for printing on non-porous substrates such as glass, plastic, metal, or ceramics, images with improved adhesion. There also exists a need for a jet ink composition that is substantially free of methanol and MEK. There further exists a need for a jet ink composition that dries fast enough for use in continuous jet ink printers. There further exists a need for an ethanol based jet ink composition that provides short drying times.
These and other objects of the present invention will be apparent from the detailed description of certain embodiments of the invention set forth below.